: The ring-opening polymerisation of lactones, by different initiator-catalyst systems, has been studied. Tin(ll) bis(2-ethylhexanoate) has been used to catalyse the polymerization of lactide with various types of alcohol initiators. 1H NMR spectroscopy was used to monitor the polymerization process. Mechanistic studies suggest that the polymerisation course, to a large extent, depends on the reactivity of the initiators. In the case of primary or secondary alcohols, e.g. poly(ethylene glycol) and methyl lactate, it is found that when the initiator concentration exceeds that of the catalyst, the number of chains formed also exceed the number of catalyst molecules. The chains are propagated through a shift of the catalyst from one chain to another. GPC chromatography clearly shows oligomer formation in the first stage of the polymerization. In the case of tertiary alcohols, it appears that the number of chains formed is comparable to the number of catalyst molecules. By choosing an appropriate initiator the reaction course and molecular weight can be controlled. A general polymerization mechanism of the ring-opening polymerisation of this type is briefly discussed.